This invention relates to a music tone pitch shift apparatus (hereinafter referred to as a "pitch shift apparatus") and particularly to one in which analog audio signals are converted into pulse code modulation (PCM) digital data and then pitch shifted.
Recently, audio signal processing techniques have undergone rapid development, and digital signal processing techniques have been developed to achieve high performance and high precision.
The pitch shift apparatus has been improved in its performance and precision by the use of the digital processing technique as the electronic musical instruments or electronic accompanist machines for vocalists (KARAOKE) have been widely used and developed. The conventional pitch shift apparatus has used an adaptive delta modulation (ADM) system as an analog to digital (A/D) conversion technique for converting analog signals into digital signals in order to reduce circuit scale and cost, and made the pitch shift process and D/A (digital/analog) conversion on the ADM digital, data to thereby produce analog audio signals (see The Journal of Institute of Electronics and Communication Engineers of Japan, EA85-40, issued 1985, 9.26).
In this conventional ADM system pitch shift apparatus, however, satisfactory performance could not be achieved. In recent years, the ADM system has almost been replaced by pulse code modulation (PCM) as the A/D conversion technique, because the signal to noise ratio (S/N) distortion, and linearity in the A/D conversion of the PCM system have been greatly improved with the development of the digital technology.
One example of the conventional PCM system pitch shift apparatus will hereinafter be described.
FIG. 3 is a block diagram of a conventional pitch shift apparatus, and FIG. 4 is an explanatory diagram for the explanation of the basic principle of the pitch shift operation, FIG. 5 is a schematic diagram useful for explaining the addresses of a ring memory in and from which writing and reading are made, and FIG. 6 is a diagram showing waveforms at the various portions of the pitch shift apparatus of FIG. 3.
Referring to FIG. 3, there are shown an A/D converter 1, a memory 2, a memory write address generator circuit (WR1 ADD) 3, a first memory read address generator circuit (RD1 ADD) 4, a second memory read address generator circuit (RD2 ADD) 5, D/A converters 9, 18, attenuators 19, 20, and an adder 21. The operation of the pitch shift apparatus will be described with reference to the drawings.
As illustrated in FIG. 3, an analog audio signal is supplied via an input terminal to the A/D converter 1, where it is sampled at a sampling frequency fs and converted into a PCM digital signal. This PCM digital signal is sequentially written in the memory 2 at the addresses specified by the memory write address generator circuit 3. The memory 2 is formed of a RAM (random access memory) as a ring memory. As shown in
FIG. 5, the address beings at 0-address, increases at the frequency fs until the maximum, and again begins at 0-address.
The first memory read address generator circuit 4 is constructed to increase the address at intervals different from those of the memory write address generator circuit 3. The timing (intervals of time) for the reading is made as follows. For example, to increase the pitch, the intervals of time are made shorter than 1/fs [sec] (write timing (interval of time)), and to decrease the pitch, the intervals of time are made longer than 1/fs [sec]. FIG. 4 shows the change of the audio signal waveform for the decrease of the pitch. From FIG. 4 it will be understood that the read timing T2 is longer than the write timing T1 (1/fs), or that the pitch-shifted waveform (b) of FIG. 4 has a frequency lower than that of the original waveform (a) of FIG. 4, or that the pitch is reduced.
The second memory read address generator circuit 5 is constructed to generate the address which is spaced by an amount corresponding to 1/2 the ring memory from the address which the first read address generator circuit 4 generates. The PCM digital data read from the address specified by the first memory address generator circuit 4 is supplied to the D/A converter 9, and the PCM digital data read from the address specified by the second memory address generator circuit 5 is fed to the D/A converter 18. The outputs from the D/A converters 9, 18 are respectively supplied through the weighting attenuators 19, 20 to the adder 21, which produces the final pitch-shifted output (analog audio signal).
In this pitch shift apparatus, however, the amplitude of the pitch-converted output is not constant (see FIG. 6e), or an amplitude-modulated analog audio signal is obtained, so that a sine wave input with a constant amplitude results in offensive sound. In other words, since the timing T1 of the address from the memory write address generator circuit 3 is different from that T2 of the address from the first and second memory read address generator circuit 4, 5, overtaking or lapping between the two addresses occurs with a constant period as time elapses. At this time, the PCM digital data read from the address specified by the first read address generator circuit 4 has discontinuous points (where the overtaking or lapping occurs) at, for example, ta, tb. tc, ... as shown in waveform (a) of FIG. 6 depending on the phase of the audio signal, and similarly the PCM digital data read from the address specified by the second read address generator circuit 5 which differs in read timing by 1/2 the ring memory has discontinuous points at intermediate points between the discontinuous points shown in waveform (a) of FIG. 6, or at ta' between ta and tb, tb' between tb and tc, ... as shown in waveform (b) of FIG. 6. In waveforms (a) and (b) of FIG. 6, for convenience of explanation, the digital data is shown in an analog manner. The PCM digital data at these discontinuous points cause impulse noise. Thus, to reduce this noise, the prior art used the cross-fade method. In this method, if the waveforms shown in (a) and (b) of FIG. 6 are expressed by F1(t) and F2(t), respectively, and the weighting coefficients of the attenuators 19 and 20 by .alpha.1(t) and .alpha.2(t), respectively, these waveform are usually weighted by the functions .alpha.1(t), .alpha.2(t) which have the relation, .alpha.1(t)+.alpha.2(t)=1 as shown by waveforms (c) and (d) of FIG. 6 so that the impulse noise can be eliminated at the discontinuous points, and that .alpha.1(t).multidot.F1(t)+.alpha.2(t).multidot.F2(t) can be obtained as the final output waveform (e) of FIG. 6. In this method, however, although the impulse noise at the discontinuous points can be eliminated, the pitch shifted output waveform (the final output waveform) has an amplitude modulated (AM) component as shown by waveform (e) in FIG. 6.